1. Field of the Invention
The present invention relates to a tire magnetization method, a tire magnetized by the tire magnetization method, a tire magnetic field detection method, a tire revolution detection signal processing method, and a tire revolution detection apparatus and, more particularly, to a tire magnetization method for magnetically detecting the revolution or revolution speed (revolution per minute (rpm)) of a wheel or tire so as to measure the speed, the distance traveled, and the like of a vehicle such as an automobile or the like on the basis of the revolution speed of the tire of the vehicle, a tire magnetized by the tire magnetization method, a tire magnetic field detection method, a tire revolution detection signal processing method, and a tire revolution detection apparatus.
2. Description of Related Art
Car navigation systems used for confirming the current position of a vehicle, navigating a vehicle, and so on have come into existence in about 1990, and have become popular.
A car navigation system has a function of detecting absolute position on the basis of radio waves from a satellite by GPS (GLOBAL POSITIONING SYSTEM) navigation. In recent years, systems using a hybrid system with a self-contained navigation that indicates the traveling condition of a vehicle on the basis of the angular deviation detected by a gyro sensor and vehicle speed data from the vehicle main body have increased and become dominant. The hybrid system can improve map matching precision.
However, in order to obtain the self-contained navigational function, vehicle speed data must be acquired from the vehicle main body. For this purpose, a user must ask an expert having the diagram of the vehicle main body for connecting the system. Such connections are hard to do by an average user, and high cost and connections requiring an expert prevents wider user of car navigation systems in the future.
The above-mentioned problems can be solved if a sensor that detects the revolution or revolution speed of a tire to measure the vehicle speed or distance traveled, and can be easily attached can be supplied. As an ideal method, it is most preferable to detect the revolution or revolution speed of the tire in a non-contact manner.
The present inventors consider the fact that steel radial tires have become popular, and such tire has a steel belt inside the outer circumferential portion. The steel belt itself has residual magnetization albeit its weak strength, and this residual magnetization radiates a magnetic field outside the tire. In fact, when the magnetic field was measured by rotating the tire once, a magnetic field distribution shown in FIG. 10 appeared. The measurement of the magnetic field was done along the outer circumferential portion at a position about 15 cm from the tire. As can be seen from FIG. 10, a clear peak is present in correspondence with one revolution of the tire, and feasibility of magnetic detection of the revolution of the tire is suggested.
However, the peak-to-peak value of the magnetic field from the tire is about 0.38 G, and is smaller than geomagnetism (about 0.5 G). In some cases, the magnetic field may become smaller than 0.1 G depending on the types of tires and the setting positions of the sensor. Hence, a very weak magnetic field from such tire must be detected with high sensitivity.
In order to satisfactorily obtain magnetic detection of the revolution or revolution speed of such steel radial tire, the following conditions must be satisfied.
i) Sensor Arrangement
The sensitivity of a magnetic sensor requires several mG in consideration of resolution. On the other hand, the sensor does not change such as a flux gate sensor.
ii) Sensor Setting
The sensor must be easy to set in a vehicle. Also, the sensor must be set at a position suitable for magnetic field detection.
iii) Removal of Influences of Disturbances
Upon tire revolution detection during actual travel of a vehicle, the magnetic fields produced by residual magnetization of iron reinforcing rods, steel frames, and the like of bridges, tunnels, and the like act as disturbances. Hence, the influence of such magnetic fields that act as disturbances must be removed.
The present invention has been made in consideration of the above situation, and has as its object to provide a tire revolution detection method and apparatus which can satisfy the above-mentioned conditions, and satisfactorily attain magnetic detection of the revolution of a tire which includes a steel belt, and a tire revolution speed detection method, which can satisfactorily attain magnetic detection of the revolution speed of the tire.
It is another object of the present invention to provide a tire magnetization method suitable for magnetic revolution detection of a tire, a tire magnetized by the tire magnetization method, and a magnetic field detection method that can obtain a stable output waveform from the tire magnetized by the magnetization method.
It is still another object of the present invention to provide a tire revolution detection signal processing method, which can absorb not only level drifts of magnetic sensor outputs due to disturbances but also variations of sensor outputs depending on the distance between the tire and sensor, and is robust against waveform distortions of the sensor outputs.
When a magnetic field pattern includes a plurality of peaks per revolution of a tire, correction of a fraction of an integer must be done upon calculating the revolution speed of the tire by counting the number of peaks, and since the individual tires have different numbers of peaks, correction values must be set in units of tires.
In view of these problems, when magnetization of the tire is controlled to obtain a signal corresponding to a predetermined number of pulses, e.g., one pulse, magnetic detection of the tire revolution is facilitated.
The present invention has been made in consideration of the above situation, and has as its object to provide a tire magnetization method suitable for magnetic revolution detection of a tire, and a tire magnetized by the tire magnetization method and, more particularly, to find out the magnetization range angle that can maximize changes in magnetic field upon revolution of the tire. It is another object of the present invention to provide a tire magnetic field detection method which can obtain a stable output waveform from the magnetized tire.